The present invention relates to plastic injection molding and more particularly to plastic injection molding using gas assist and the formation of hollow rib members on plastic injection molded parts.
In the plastic injection molding art, the usual challenges facing the product designer include designing an article having the requisite strength for the product application and satisfactory surface finish, as well as avoiding excessive weight, surface distortions, and increased cycle time. For flat or thin products, it is typical to include one or more rib members in the design to provide relative strength and structure for the molded article. The rib members are typically thicker than the molded article which increases the weight, material usage, and cycle time of the article, and often induces sink marks and other surface defects due to a thermal gradients in the area of the thickened section.
It is known in the plastic molding art to use pressurized gas, such as nitrogen, in conjunction with plastic injection molding of articles. Pressurized gas serves several purposes. The gas allows the article or rib structure to have hollow interior portions which result in savings in weight and material, thereby reducing costs. The pressurized gas also applies an outward pressure to force the plastic against the mold surfaces while the article solidifies. This helps provide a better surface on the molded article and also reduces or eliminates sink marks and other surface defects. The use of pressurized gas also reduces the cycle time as the gas is introduced and/or migrates to the most fluent inner volume of the plastic and replaces the plastic in those areas which would otherwise require an extended cooling cycle. The pressure of the gas pushing the plastic against the mold surfaces further increases the cooling effect of the mold on the part, thus solidifying the part in a faster manner and reducing the overall cycle time.
Where the rib members or other portions of the article in which the gas is being introduced are elongated, it is often difficult to provide a satisfactory molded article. For example, if the pressure of the gas is too great as it enters the mold cavity, there is a risk that it may rupture or blow out the plastic within the mold cavity, i.e. the gas is not contained within the plastic. Also, it is often difficult to have the gas migrate along the full length of an elongated, thicker plastic section, thus creating a product which has an uneven thickness and cooling cycle.
One manner which has been developed in order to overcome some of the above-mentioned problems is shown in U.S. Pat. No. 5,098,637. In that process, a secondary cavity (a/k/a “spillover” cavity) is provided at one end of the molded part or elongated rib member in order to collect and contain the more fluent plastic material which is forced out of the article or rib member by the pressurized gas.